Question

Explain how caffeine acts as a diuretic. Be sure to demonstrate an understanding of how the movement of water is affected.

Answer 1

Caffeine is a central nervous system (CNS) stimulant of the methylxanthine class. It is the world's most widely consumed psychoactive drug. Unlike many other psychoactive substances, it is legal and unregulated in nearly all parts of the world. There are several known mechanisms of action to explain the effects of caffeine. The most prominent is that it reversibly blocks the action of adenosine on its receptors and consequently prevents the onset of drowsiness induced by adenosine. Caffeine also stimulates certain portions of the autonomic nervous system.

Caffeine is an alkaloid which is present in beverages, food and drugs. Caffeine competitively antagonizes the adenosine receptors (AR), which are G protein-coupled receptors largely distributed throughout the body, including brain, heart, vessels and kidneys. Caffeine consumption has a well-known diuretic effect.

The homeostasis of salt and water involves different segments of the nephron, in which adenosine plays complex roles depending on the differential expression of adenosine receptors. Hence, caffeine increases glomerular filtration rate by opposing the vasoconstriction of renal afferent arteriole mediated by adenosine via type-1 adenosine receptors during the tubuloglomerular feedback. Caffeine also inhibits Na (+) reabsorption at the level of renal proximal tubules. In addition, caffeine perturbs the hepatorenal reflex via sensory nerves in Mall's intrahepatic spaces. Here, we review the physiology of caffeine-induced natriuresis and diuresis, as well as the putative pathological implications.